This program includes several interrelated projects, each designed to assess parameters which influence growth and/or neoplastic transformation in vitro. (1) The effects of inoculum size and a rigid subculture schedule have been tested as factors which influence neoplastic transformation. All cell strains, irrespective of the culture conditions used, became tumorigenic for syngeneic animals. Tumorigenic capacity in vivo best correlated with (a) susceptibility of the cells to activated macrophages, and (b) cytologic alterations pathognomonic of neoplastic transformation. (2) In studies designed to optimize clonal growth conditions for non-neoplastic cells, equal or better plating efficiencies have been consistently obtained with a gas phase of 1% v/v O2 as compared with atmospheric oxygen (18-20% v/v). This enhanced growth is seen with cell derived from adult as well as fetal or embryonic tissues, and an inverse relationship was observed between plating efficiency and medium PO2. (3) The chemical characterization of a low molecular weight, autoclavable growth factor for mammalian cells suggests a) the active component(s) is a peptide and b) does not function as a nutrient source.